The production of steel normally comprises the steps of converting iron ore to pig iron using a blast furnace, and thereafter converting the pig iron into steel using an open hearth furnace or a converter. Such a traditional method requires large amounts of energy and large-scale equipment, and has a high cost. Therefore, for a small-scale steel-making operation, a method comprising the steps of directly converting iron ore into raw materials used in the steel-making furnace, and converting the raw material into steel using an electric furnace and the like has been used. With respect to direct iron making, a direct reduction process has been used to convert iron ore into reduced iron.
However, the reduced iron produced by a direct reduction process is highly reactive and reacts with oxygen in the air to generate heat. Therefore, it is necessary to seal the reduced iron with an inert gas, or by some other measure, during transportation and storage of the reduced iron. Accordingly, iron carbide (Fe.sub.3 C) containing a comparatively high iron (Fe) content, and which has a low reaction activity and can be easily transported and stored, has recently been used as the iron-containing material for steel making in an electric arc furnace and the like.
Furthermore, an iron-making or steel-making material containing iron carbide as the main component is not only easy to transport and store, but also has the advantage that the carbon combined with iron can be used as a source of fuel in an iron-making or steel-making furnace, and can be used as a source to generate microbubbles which accelerate the reaction in the steel-making furnace. Therefore, materials for iron making or steel making containing iron carbide as the main component recently have attracted special interest, as set forth in publications 1, 2, and 3 listed hereafter.
According to one method of producing iron carbide, iron oxides (e.g., hematite (Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4), wustite (FeO), etc.) in iron ore are reduced and carburized in a single process. A "single process" means an operation performed by simultaneously introducing a reducing gas and a carburizing gas to a single reactor, as shown in the following reaction formulas (1)-(6). Fine-sized iron ore is charged into a fluidized bed reactor and is reacted with a gas mixture comprising a reducing gas (e.g., hydrogen gas) and a carburizing gas (e.g., methane gas and the like) at a predetermined temperature. EQU 3Fe.sub.2 O.sub.3 +H.sub.2 .fwdarw.2Fe.sub.3 O.sub.4 +H.sub.2 O(1) EQU Fe.sub.3 O.sub.4 +H.sub.2 .fwdarw.3FeO+H.sub.2 O (2) EQU FeO+H.sub.2 .fwdarw.Fe+H.sub.2 O (3) EQU 3Fe+CH.sub.4 .fwdarw.Fe.sub.3 C+2H.sub.2 (4) EQU Fe.sub.3 O.sub.4 +CH.sub.4 +2H.sub.2 .fwdarw.Fe.sub.3 C+4H.sub.2 O(5) EQU 3FeO+CH.sub.4 +H.sub.2 .fwdarw.Fe.sub.3 C+3H.sub.2 O (6)
The overall reaction of equations (1) through (4) and (5) and (6) is set forth in reaction formula (7): EQU 3Fe.sub.2 O.sub.3 +5H.sub.2 +2CH.sub.4 .fwdarw.2Fe.sub.3 C+9H.sub.2 O.(7)
Further, prior art in the field of the present invention is described, for example, in the publication of the Japanese translation of International Patent Application No. 6-501983 (PCT/US91/05198), including the operation report of industrial equipment in publication 11 listed hereafter, and those described in publications 4, 5, and 8 listed hereafter. In addition, German Patent No. 4320359 discloses a process provided with two-stage reactors.
The carburization process also can be accomplished by using carbon monoxide (CO) as the carburizing gas component as set forth in the following reaction formula (8). EQU 3Fe+CO+H.sub.2 =Fe.sub.3 C+H.sub.2 O (8)
Prior art in the above-described field further is described in publications 6 and 7 listed hereafter.
TABLE 9 ______________________________________ Publications ______________________________________ 1 "TEKKO KAI," July, 1993, pp. 40-44. "Iron Carbide, iron source attracting interest" 2 "The potential for use of iron carbide as an electric furnace raw material," 16th Advanced Technology Sympo- sium ISS-AIME Alternate Iron Sources for Electric Arc Furnace, May 2-5, 1993, Myrtle Beach. 3 A. W. Swanson, "Iron Carbide, a possible replacement for premium quality scrap," Preprint 93-28, presentation at the SME Annual Meeting, Reno, Nevada, February 15-18, 1993. 4 F. V. Povoa, "Role of iron are supplier in scrap substi- tute process development," Iron & Steel Scrap, Scrap substitutes direct steel-making, March 21-23, 1995, Atlanta Georgia. 5 Nakagawa et al., "Influence of the nature of iron ore on the formation of cementite," CAMP-ISU Vol. 7 (1994)- 85. 6 Hayashi et al., "Formation of iron carbide from iron ore using fluidized bed (Production of iron carbide-2) CAMP-ISU Vol. 8 (1995)-111. 7 Hayashi et al., "Formation of iron carbide from iron ore (production of iron carbide-1), CAMP-ISU Vol. 8 (1995)-110. 8 Nakagawa et al., "Influence of gas composition and temperature on formation of cementite, " CAMP-ISU Vol. 8 (1995)-109. 9 Mori et al., "New iron making process and fluidized bed," Chemical Equipment, June, 1986, pp. 99-108. 10 T. P. McAloon, I&SM, February, 1994. 11 33 Metal Producing, January, 1995 (pp. 36, 37 & 49). ______________________________________
The above-described conventional methods have the following disadvantages.
Regarding conventional methods described in the publication of the Japanese translation of International Patent Application No. 6-501983 (PCT/US91/05198), and publications 3 and 5, which use present-day industrial equipment, the iron-containing material for iron making contains at least one, or a mixture of two or more, iron oxides, such as hematite, magnetite, and wustite, and iron hydroxides, such as ferrous hydroxide and ferric hydroxide, as the main component, e.g., iron ore or dust and the like generated from iron-making processes. The process of removing oxygen combined with iron atoms of the iron-containing material for iron making uses methane (CH.sub.4) as a component in a carburizing reaction gas to convert the iron-containing material into iron carbide (also termed cementite and Fe.sub.3 C herein) in a single reactor at a temperature of about 600.degree. C. using a gas mixture containing methane. The gas mixture is suitable for the carburizing reaction, that is, the main reaction (i.e., reduction and carburization) is performed in a single process. Because methane (CH.sub.4) in this process is used directly as the carburizing gas, the carbon atom in methane acts as the carburizing component and the hydrogen atoms in methane act as the reducing component. This process has the advantages that the amount of H.sub.2 (hydrogen gas) and CO (carbon monoxide) consumed is small and the apparatus is simple. However, the following disadvantages also are apparent.
Because the reaction is a catalytic reaction between a solid iron oxide and reducing and carburizing gases, the reaction speed is slow and the reaction time (i.e., a time required for a complete conversion to the desired iron carbide product) is long, thereby requiring a long time to obtain a predetermined amount of the steel-making material compared to a conventional iron-making method, such as a blast furnace process and the like. Therefore, it is necessary to enlarge the scale of the equipment in order to increase production per unit time. As a result, the main objectives of the direct iron-making method, which are to decrease equipment scale and production costs compared to traditional iron-making methods, are not met.
The reaction temperature preferably is increased in order to increase reaction speed. However, in a reducing reaction of iron oxide, when the reaction temperature is increased to about 600.degree. to 700.degree. C., even though this temperature is below the melting point of iron oxide, the angular surface of an iron oxide crystal becomes smooth due to surface tension (referred to as sintering or semi-melting, hereafter termed "sintering") as the reduction ratio approaches 100%. This phenomena results in a loss of reaction activity. As illustrated in FIG. 8, and in publication 9, a graph of the relationship between reaction temperature and reaction time in the reducing reaction of hematite shows that the reaction time increases in the range of about 600.degree. to 700.degree. C. when about 100% reduction is approached.
Therefore, the objective of increasing reaction speed is not achieved, even if the reaction temperature is increased. When a large amount of water is generated in the reducing reaction, or when the raw material does not flow smoothly due to a structural feature of the reactor, the water reacts with the iron ore to cause local solidification, that is, a so-called "sticking phenomenon" occurs. When sintering or sticking occurs, the iron oxide particles condense or agglomerate and, therefore, become impossible to remove mechanically.
Furthermore, the reducing reaction shown in above reaction formulas (1) through (3), and the carburizing reaction shown in above reaction formula (4), are performed in a single process by contacting iron oxide with a gas mixture containing hydrogen, methane, and the like. Therefore, both the reducing and carburizing reactions must be considered, and the composition of the reaction gas and the reaction temperature cannot be independently set to optimize the reducing and carburizing reactions, respectively.
Therefore, the amount of reaction gas (i.e., the amount of a reaction gas to be contacted in order to produce a unit amount of product) is increased. As shown in Table 8a described hereafter, the amounts of energy, electrical power, cooling water, and the like consumed in a conventional iron carbide producing process described in the publication of the Japanese translation of International Patent Application No. 6-501983 (PCT/US91/05198) are greater than the corresponding amounts used in a conventional direct reduction iron-making process (e.g., the MIDREX process, etc.).
The method described in German Patent No. 4320359 which reduces the amount of the reaction gas, completes the reaction in two stages. In the first stage, iron ore is partially reduced using a mixture of a 50 to 20% equivalent amount of a circulating reaction gas, and a 50 to 80% equivalent amount of a remaining, partially reacted, circulating reaction gas discharged from the second stage. Then, the partially reduced iron ore is transferred to the second stage, and a further reduction and carburization is conducted using 50 to 80% equivalent amount of the circulating reaction gas. An object of the described method is to enhance the reaction efficiency of the gas by bringing 50 to 80% of the circulating reaction gas into contact with the ore in two stages. The described method has accomplished some reduction in the consumption of reaction gas compared to the conventional method described in the publication of the Japanese translation of International Patent Application No. 6-501983 (PCT/US91/05198), or publication 11, which describes present-day industrial equipment.
However, the following disadvantages can be considered. In particular, the circulating reaction gas introduced into the second stage requires a sufficient carbon potential (i.e., chemical reaction force) to produce iron carbide. Therefore, it is necessary to increase the methane concentration of the reaction gas. Accordingly, the concentration of hydrogen gas as the reducing component in the reduction gas is relatively low. Also, because the gas after the completion of the reaction in the second stage contains a high water and carbon dioxide content as products of the reducing reaction, the reduction capability of the gas is lowered. Therefore, the reaction time in the first stage is increased. Accordingly, a reduction in the total reaction time for the first and second stages cannot be achieved.
The gas composition and temperature of the first and second stages cannot be independently set, and, therefore, the reduction ratio or the metallization ratio, and the carburization ratio cannot be independently controlled.
The processes disclosed in publications 6 and 7 use carbon monoxide (CO) gas as the carburizing reaction gas component as described above, and a considerable reduction in reaction time of the carburizing reaction is reported. However, a comparison between the overall reaction formula to fully convert hematite into iron carbide when using carbon monoxide (CO) or methane (CH.sub.4), respectively, as the carburizing gas, must be considered.
In case of CO, the overall reaction formula is as follows: EQU 3Fe.sub.2 O.sub.3 +2CO+11(H.sub.2 and/or CO)=2Fe.sub.3 C+11(H.sub.2 O and/or CO). (9)
In case of CH.sub.4, the reaction formula is as follows: EQU 3Fe.sub.2 O.sub.3 +2CH.sub.4 +5(H.sub.2 and/or CO)=2Fe.sub.3 C+9(H.sub.2 O and/or CO.sub.2). (10)
As readily illustrated in reaction formulas (9) and (10), when carbon monoxide is used as the carburizing reaction gas component, it is necessary to supply 2.6 times ((2+11)/5) the amount of a gas mixture of CO and H.sub.2 compared to using methane. H.sub.2 and CO are produced industrially by bringing a natural gas containing methane (CH.sub.4) as a main component into contact with steam in the presence of a catalyst at a high temperature and under high pressure, followed by catalytic reaction (referred to as steam gas reforming process). Accordingly, when carbon monoxide is used as the carburizing gas, an expensive steam gas reforming unit is further required and energy consumption increases. This relationship is shown in FIG. 10.
The present invention solves the above-described disadvantages of the conventional method of producing Fe.sub.3 C. A main objective of the present invention is to provide a method and apparatus for producing iron carbide efficiently and economically. The method and apparatus are capable of shortening the reaction time, reducing consumption of reaction gas and energy, and enabling use of smaller size equipment.
These objects, as well as other objects and advantages of the present invention, will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.